Which legacy devices are still at risk when PSTN/POTS lines are retired or moved to VoIP?

The devices at risk share one trait: they were built to send or receive information as precise audio tones over a copper line, and they assumed that line would always be powered and electrically clean.[2] A normal phone call survives a dropped syllable because a listener fills the gap, but a fire panel or a fax machine reads every part of the waveform, so the same small imperfection that voice ignores turns a machine-to-machine signal into an error.[9] When a carrier moves one of these lines to Voice over IP, the audio gets compressed and packetized, and that is enough to corrupt or lose the exact tones these devices depend on.[5][9]

The second weakness is power. A traditional copper local loop carried a low voltage supply from the telephone exchange, backed by batteries at the central office, so a corded phone and the equipment sharing its line usually kept working through a blackout.[2][11] A VoIP or fiber line cannot carry that power, so the handset, the router, and anything wired behind them stop the moment the building loses electricity, unless someone has added a battery.[11] For a medical pendant or an alarm dialer, that outage gap is the whole reason the device exists.

The categories that need attention

Six kinds of equipment come up again and again on POTS lines: fire alarm panel communicators, elevator emergency phones, security and burglar alarm systems, medical alert units, fax machines and payment terminals, and a mixed group of other analog endpoints such as entry intercoms, teletypewriter (TTY) devices, and remote telemetry units.[10][3] Most of them are life-safety or money-movement systems governed by codes or card-network rules, which is why a silent failure matters more than a dropped voice call.[3][6] The table below groups them, and the sections after it explain what each one does, why it breaks, and the practical fix.

Timing is what makes this urgent rather than theoretical. Carriers are decommissioning copper on their own schedules, and in the United States the written warning before a line is cut can be measured in weeks, not the long runway many owners assume.[1][10] The clearest hard deadline is in the United Kingdom, where the analog network is being switched off and most lines are moving to VoIP by January 2027.[11]

Fire alarm control panels report to a central station through a Digital Alarm Communicator Transmitter, or DACT, which seizes the phone line and dials out when there is an alarm or a trouble condition.[4] The older approach used two dedicated copper POTS lines with automatic supervision, and the panel raises a trouble signal if either line fails.[4] That supervision is the part VoIP tends to break. When a carrier quietly migrates the line to a VoIP or managed voice product, the panel often cannot confirm its path, which shows up as persistent phone-line trouble signals that will not clear, and a real alarm may not reach the station.[4][5] Traditional alarm tones were designed for an analog circuit, so once they are converted into digital packets they can arrive in a form the monitoring center does not recognize.[5] NFPA 72 does not treat ordinary VoIP as acceptable for this job.[4] The common fix is a listed IP or cellular communicator, often dual-path for redundancy, or a service specifically engineered to emulate an analog line so the DACT still sees the supervision it expects.[4][5]

An elevator emergency phone connects a trapped passenger to a monitoring center, and ASME A17.1, with CSA B44, requires a working two-way voice link regardless of the line technology behind it.[6] For decades that link rode on a copper POTS line, and the code also requires the system to verify automatically, at least once a day, that the line is operational and to signal a fault if it is not.[6] When the copper is retired, an emergency phone that has no compatible replacement can simply go silent. A dead line here is an immediate code violation and a life-safety hazard rather than an inconvenience, because the building's obligation does not retire along with the carrier's copper.[6] Standard consumer or office-grade VoIP often cannot guarantee the daily line verification, the automatic fault signaling, or the hours of backup power the use case assumes, which is why purpose-built cellular and code-listed IP solutions are what most buildings move to.[6]

Intrusion and burglar alarm panels report to a central station by capturing the phone line and sending short bursts of tones in a format such as Contact ID.[5] Those tones are exact, and VoIP can change or drop them: jitter, packet loss, and codec translation can leave the central station never receiving an alarm, or receiving one that is corrupted or routed to the wrong account, which also produces false alarms.[5] Some panels even register that they reported successfully when nothing actually reached the monitoring center, which is the failure mode that hides until an incident exposes it.[5] The panel itself keeps working locally, so the siren may still sound; the part that breaks is the quiet hand-off to the people who dispatch help. The practical fix is an IP or cellular alarm communicator that carries the signal as data instead of as analog tones, or a service built to emulate the old line for panels that cannot be replaced right away.[5][3]

In-home medical alert systems, also called personal emergency response systems, work by having a base station dial a response center when the wearer presses a pendant, and most in-home units were designed around a standard analog landline.[7] Analog lines are generally less prone to the signal drops that affect internet and cellular paths, which is one reason they were long treated as a dependable choice for this kind of equipment.[7] A base unit can run over VoIP if it is labeled compatible with digital phone service, so this is less an outright block than a reliability question, but two risks stay in place: the internet path is less consistent than copper, and it goes dead in a power outage, which is often the exact moment a person needs to call for help.[7][11] The safer route for most households is a unit with its own cellular connection and battery, or, if a home line is kept, confirming digital compatibility and adding backup power so the device still works when the electricity does not.[7][11]

Fax machines and older credit-card and point-of-sale terminals move data as modem tones over the voice line, and that is what makes them fragile on VoIP.[9][10] Voice codecs such as G.729 discard audio detail the ear will not miss, but that discarded detail is the signal, so the data arrives mangled; even a G.711 pass-through breaks the moment there is packet loss or jitter, because a single lost packet can be a hole in the page or the transaction.[9] For fax, the recognized answer is ITU-T Recommendation T.38, which understands the fax session and carries the page as data with fax-aware redundancy rather than trying to keep the sound intact across the network.[8][9] Card terminals and other dial-up devices hit the same modem problem, so many merchants move them onto an IP or cellular terminal instead of trying to nurse an analog transaction across a packet network.[10] The tell is usually intermittent failure: pages that stop halfway, or transactions that time out under load while a plain voice call on the same line sounds fine.[9]

A long tail of quieter devices also lives on POTS lines: gate and entry-door intercoms that dial a phone to buzz visitors in, teletypewriter (TTY) devices used by people who are deaf or hard of hearing, and remote telemetry or monitoring units at utilities and unattended sites.[10] They share the same two weaknesses as the higher-profile systems, because their tones or modem signals do not survive compression and loss well, and they lose the exchange power that kept them running through an outage.[9][2] Being easy to forget is their real risk. These lines are often the last ones discovered still on copper when a carrier issues a retirement notice, so an inventory of every analog line in a building, and of what actually sits on each one, is the practical first step before any of them goes dark.[10][3]

Two properties of a copper POTS line explain almost every one of these failures. The line was a clean, low-latency, always-on analog circuit, so tones and modem signals crossed it intact, and it carried its own power from the exchange, so it survived a blackout.[2][11] A VoIP path breaks both assumptions, because it compresses and packetizes audio and it depends on local power and a working internet connection.[5][11] That is why voice calls can sound fine on the very line that quietly kills a fax or an alarm report: a person tolerates a dropped fraction of a second, but a machine reading exact tones does not.[9]

What to check before a line is cut

Start by inventorying every analog line in the building and identifying what sits on each one, since the risky devices are usually the ones nobody thinks about until they stop working.[10][3] For each device, confirm that the replacement path is a listed or code-compliant option for that specific use, because life-safety systems such as fire panels and elevator phones have requirements that ordinary VoIP does not meet.[4][6] Confirm backup power for anything that must work in an outage, and test the device end to end after any cutover, which means sending a real signal to the monitoring center rather than settling for a dial tone.[5][11]

Not every phone or device on the old network is in trouble, and a few distinctions separate real risk from noise.

An ordinary voice phone is not the problem

A desk or cordless phone used for talking generally moves to VoIP without special work, because voice is exactly what the packet network is tuned to carry.[1] The at-risk devices are the ones that send data or reach for help automatically, not the ones a person speaks into.

Managed voice is not always a safe analog substitute

Carriers sometimes shift a customer onto a VoIP or managed voice product that looks like a phone line but does not meet alarm supervision rules, which can leave a fire or security panel non-compliant even though people can still place calls on it.[4][5] A service specifically engineered to emulate an analog line can be listed for these uses, while the ordinary consumer VoIP product usually is not.[4]

Cellular and IP replacements are not interchangeable for every device

A cellular communicator suits panels and dialers that only need to reach a monitoring center, while some sites prefer a dual-path IP and cellular device for redundancy, so the right choice depends on the code that governs the device and the connectivity already in the building.[5][6]

Retirement is not optional or distant

The copper is going away on the carrier's timeline rather than the customer's, and the notice before a cut can be short.[1][10] Treating the migration as a planned project tends to go better than discovering a dead alarm or elevator line during an inspection.[3][6]